Boxelder bugs
Boxelder bugs are native true bugs, about 1/2 inch long, black with red markings. They feed mainly on the seeds of female boxelder trees, and on other maples and ash, without doing noticeable harm to the trees. They do not sting or spread disease, and bites are rare: Vermont says they do not bite, while Colorado State notes rare reports of bites. They are a nuisance because the adults gather in large numbers on sunny walls in late summer and autumn and get indoors to spend the winter. Outdoors, nothing needs controlling. The answer is exclusion: seal gaps before the end of August, and vacuum or sweep up any that get inside.
Boxelder bugs (Boisea trivittata) gathering on a south-facing wall in autumn. Photo: pending.
Identification
Adults are about 1/2 inch (12 mm) long, black with orange or red markings, and have three red stripes on the section just behind the head. The wings lie flat over the body and overlap to form an X, with red edges along their base, and the abdomen under the wings is bright red. The eggs are glossy rusty red. Newly hatched nymphs are tiny (about 1/16 inch) and bright red, and turn red and black as they grow.
The University of Vermont shows three milkweed bugs as lookalikes: the large milkweed bug, the small milkweed bug and the false milkweed bug. Colorado State adds that the small milkweed bug resembles the boxelder bug. Compare the markings closely. A boxelder bug has three thin red stripes behind the head and red edges on its wings, and it is the one that gathers on house walls in autumn, especially near boxelder trees.
Easily confused with
- Red-shouldered bugHarmlessThe red-shouldered bug's red is on the shoulder edges and the rim of the abdomen; the boxelder bug has three red stripes down the thorax.
- Brown marmorated stink bugAlso a pestBMSB is a marbled brown stink bug with two white bands on each antenna; boxelder bugs are black with red stripes.
- Milkweed bugsHarmlessThe boxelder bug has three red lines on the thorax, a red line down each side and a diagonal red line on each wing.
Damage symptoms
Plant damage is minimal. Extension entomologists at Minnesota and Vermont report no noticeable feeding injury to boxelder, maple or ash. The problem is the numbers. In late summer and autumn the bugs cluster on warm walls, then work in through gaps around windows, doors, vents and utility lines. Indoors they sit in wall voids and attics and come out on mild winter days and in spring. They normally do no damage to the house itself. Their droppings can stain curtains, walls and siding, and they smell unpleasant when crushed.
- Dozens to thousands of black-and-red bugs on the sunny south or west side of the house in late summer and autumn
- Bugs appearing at windows and on walls indoors on warm days through winter and early spring
- Small stains on curtains, walls or siding from their droppings
- An unpleasant smell when they are crushed
- Bright red nymphs and adults on the trunks and seeds of female boxelder trees in summer
Lifecycle & degree days
Only adults survive the winter, sheltering in tree crevices, wall voids, attics and garages. In spring they fly to host trees, usually boxelder, and lay their eggs there. From mid-July they move to female, seed-bearing boxelders to lay eggs. New Hampshire describes a second generation that matures in the autumn, and Vermont reports several generations a year in northern New England. The last generation gathers on sunny structures in autumn, and the bugs become inactive once the weather turns cold. Numbers tend to be higher after a warm spring and a hot, dry summer. They can fly a couple of miles or more to find a winter site.
- Egg hatch
- 7–14 days (sources differ)
- Nymphal stages
- 5
- Generations / yr
- 1–2, or several in New England (sources differ)
- Overwinters
- Adults only, in crevices and buildings
- Seal the house by
- End of August
Host crops
Boxelder bugs feed on the seeds of female boxelder trees (Acer negundo) above all, and also on other maples and ash. In heavy years they may feed on raspberry and strawberry fruit, but the damage is not considered serious. The trees are listed here as places the bugs come from, not as plants at risk. Minnesota and Vermont do not recommend cutting down or spraying boxelders to get rid of the bugs: the trees are common, and the bugs can fly a couple of miles or more to find a winter site. Vermont does note that removing a boxelder growing within a few yards of the house may reduce numbers somewhat.
Monitoring & action thresholds
New Hampshire Extension says insecticide sprays are generally not recommended. Minnesota says that where numbers are large or invasions recur, an insecticide around the outside of the house can supplement sealing, best applied in late summer and early autumn as the bugs first cluster, but says that spraying boxelder trees or removing them is not a good way to manage the bugs. Once the bugs are indoors, Minnesota does not recommend insecticides, because the bugs that emerge indoors live only a few days.
There is nothing to count. The date to watch is late summer. If the bugs gathered on your walls last autumn, make the repairs before the end of August, which is Minnesota's deadline (New Hampshire says before 10 September). They favour the warm south and west walls, so check and seal those sides first.
Biological control
Nothing to release. Boxelder bugs do no real harm outdoors, so there is nothing to control there. Buying a predator will not keep them out of the house. Late in their development they may themselves eat other soft-bodied insects.
Cultural controls
Exclusion is the answer. Everything else is tidying up after the ones that got in.
- Before the end of August, repair window and door screens, fit door sweeps, and screen attic and roof vents
- Seal around pipes, cables and other utility entry points, and seal any cracks or openings of 1/8 inch or more in outside walls
- Indoors, vacuum them or sweep them into a dustpan. Kill them before emptying the bag or they escape. Avoid crushing them: they smell and can stain fabric
- Outdoors, spray clusters with soapy water or hose them off walls and trunks with a forceful jet. Soap has no lasting effect, so repeat as they gather
- Clear woody debris and dense shrubs around the house where they shelter
- Do not spray or cut down boxelder trees to get rid of them. The bugs fly in from elsewhere
Chemical, last resort
Sources
University of Minnesota Extension, Boxelder bugs (Hahn & Ascerno, reviewed 2023)
University of Vermont Extension, Boxelder Bugs, Boisea trivittata (M. Skinner, updated Oct 2022)
UNH Extension, Boxelder Bug, Pest Fact Sheet 54
Colorado State University Extension, Boxelder bugs, fact sheet 5.522
Page status
Drafted from the sources below, then checked claim by claim against those sources on 23 September 2026.
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